1. Field of the Invention
The present invention relates generally to filters that selectively pass and attenuate electromagnetic waves and, more particularly, to low pass filters for attenuating high frequency electromagnetic signals.
2. Description of the Prior Art
Various structures commonly known as “filters” are used for suppressing or attenuating, to a desired specification, electromagnetic waves impinging on and propagating through the filter, depending on the signal's or wave's constituent frequencies. The number and scope of fields of communication, entertainment, and industrial equipments and systems requiring electronic filters is essentially indefinable. Therefore, it will be understood that the example applications for the filter described herein are not limiting; the fields are presented to assist the person of ordinary skill better understand the present filter, and to make and use a filter in accordance with described herein, for either an application similar to the example application, or any other of a wide range of applications.
Textbooks, technical journals, and other publications embody a large knowledge base of filters, including their types, structures, guidelines for selection, methods of design, construction, and testing. Within this large existing knowledge base, it is also well known that problems exist in designing and constructing a “low pass” filter, i.e., a filter that attenuates electrical signals above a “cut-off” frequency while, at very high frequencies, both maintaining a given characteristic impedance and adequate attenuation. It is also known that problems exist in designing and/or constructing a filter that meets such impedance and attenuation criteria while operating at very low temperatures.
Stated in reference to particular example requirement, in the existing art of electronic filters it is difficult to construct a low pass filter that can operate at temperatures such as, for example, 4 degrees Kelvin, provide a characteristic impedance of, for example, 50Ω, and provide, for example, −80 dB of attenuation for frequencies above a cutoff frequency of, for example, 100 MHz, while maintaining that attenuation for signals having components over, for example 5-10 GHz.
For purposes of this description, the terms “signal” and “electrical signal” will mean, unless otherwise clear from the context, any electromagnetic energy propagating through, or coupling between, any medium or structure, regardless of informational content in the signal. In other words, the phrases “signal” and “electrical signal” include electromagnetic energy that, for the intended purposes of the invention, are noise, including white noise, or other energy that the filter is intended to attenuate, i.e., not pass.
Further, the phrase “characteristic impedance” is very well known in the electronic filter art and, therefore, further description is omitted except where it is helpful for an understanding of this invention.
An example that reveals certain shortcomings in the prior art of electronic filters is presented by systems and equipment used in research, development and, eventually, manufacture of quantum computers. The present invention is not directed to quantum computing per se. The present invention is a novel method and apparatus for low pass filtering that, in addition to other likely benefits, has very good high frequency attenuation, can be easily built to meet impedance matching requirements, and maintains these attenuation and impedance characteristics at low temperatures. Present and anticipated future quantum computing machines are one, but not the only, system that would benefit from such a filter. However, it is not necessary to describe the theory of quantum computing theory in order to enable construction of a working embodiment of, or to otherwise practice, the invention. Quantum computing methods, equipment and systems are described only where necessary to better understand the example filters described herein, and to assist the user in selecting dimensions, materials and arrangements that fit the user's particular requirements.
In the example field of quantum computing, it is known that decoherence in superconducting qubits is often caused by high frequency noise transmitted along electrical leads connecting the qubit to measurement electronics at room temperature. The term “qubit” is known in the art quantum computing and further description is omitted, as it is not necessary for understanding this invention. One kind of noise comes directly from the measurement electronics at room temperature. In this case the filter can be located anywhere between the measurement electronics and the qubit. The second type of noise is Johnson (“white”) noise that is produced by resistive elements in the electrical connections between the room temperature electronics and the qubit. The location of these resistive elements will usually determine where one or more filters need to be thermally well grounded at one or more carefully chosen temperatures. For purposes of this description, the phrase “thermally well grounded” means a temperature difference of less than approximately 10%, using cooling and connection methods that are well known in the art of low temperature technology.
As an illustrative example of such temperatures, a qubit can be measured in a dilution refrigerator, which attains a typical minimum temperature of about 20 millidegrees Kelvin (“mK”), measured at the mixing chamber within a vacuum can that is immersed in liquid He4, itself at a temperature of 4.2 degrees Kelvin. Before reaching the qubit, all electrical wiring is preferably thermally grounded at, for example, approximately 4.2° K, 1.3° K, 0.7° K, and 0.1° K. These are example temperatures of operating parts of the dilution refrigerator that can handle a sizeable heatload, i.e., the electrical wiring, at that temperature.
There are known methods and structures directed to filtering unwanted noise having frequencies above, for example, 1 MHz at low temperatures. All have shortcomings either in terms of impedance or frequency characteristics. One example is a miniature thin film filter as reported by Vion et al., J. Appl. Phys. 77, 2519 (1995). Another example is a distributed thin film microwave filter reported by Jin et al. Appl. Phys. Lett. 70, 2186 (1997). Still another example is the Philips Thermocoax filter, as discussed in A. Zorin, Rev. Sci. Instrum. 66, 4296 (1995). In most cases these filters were first used to reduce noise in single electron tunneling experiments. Perhaps the simplest and easiest to fabricate “microwave” filter is the bulky metal powder filter. The metal powder filter was first discussed in more detail by Martinis et al., Phys. Rev. B 35, 4682 (1987) and subsequently developed and discussed in detail by others. See K. Bladh et al. Rev. Sci. Instrum. 74, 1323 (2003), and A. Fukushima et al., IEEE Trans. Instrum. Meas. 45, 289 (1997).
The metal powder filters known in the relevant art have a central conductor that is surrounded by metal powder or a metal powder/epoxy mixture. The filter attenuates an incoming electrical signal via eddy current dissipation in the metal powder. The known art teaches, however, that the central conductor is shaped into the form of a spiral to increase the attenuation. This does indeed increase the attenuation but, as observed by the present inventors, these spiral conductor metal powder filters cannot be designed to have a characteristic impedance near 50Ω at high frequencies. The present inventors have identified that such filters cannot provide a 50Ω impedance at high frequencies because each adjoining loop of the spiral is capacitively coupled to the next loop, and if the spiral is “tight” then at high frequency this coupling looks like a short between loops. Stated differently, the physical design of known metal powder low pass filters creates what is technically a short at high frequencies, not 50 ohms.
In many high frequency applications, however, it is necessary to have an all matched 50Ω impedance measurement setup. If low pass filters are used they also must be 50Ω. The known metal powder filters cannot, because of their spiral form, meet this requirement.